The aim of the present study is to evaluate the effect of mental stress on the initiation and termination of ventricular arrhythmias in patients who have received implantable cardioverter-defibrillators (ICDs) for clinically significant ventricular arrhythmias. Following implantation of an ICD, patients undergo routine outpatient device testing programming, during which their arrhythmias are induced using non-invasive programmed stimulation to determine optimal device settings. In this study protocol, following routine determination of electrophysiologic parameters and vetricular tachycardia (VT) induction, the programmed stimulation protocol will be repeated while the patient undergoes a mental stress protocol administered by a psychologist, to determine changes in electrophysiologic parameters and VT induction and termination. To reflect the level of stress induced, plasma catecholamine levels will be drawn at baseline and during stress. To determine a possible role of silent ischemia in stress- induced arhythmias, left venticular (LV) function will be monitored concurrently using an ambulatory left ventricular function monitoring device.